To cool electronic devices, frequently several fans are operated in parallel in a plug-in unit or the like, for example in a so-called “rack” for servers or in the switch cabinet of a ground station for cell phones. The use of the plurality of fans serves to provide a degree of redundancy, i.e. of one of the fans fails, then the remaining fan or fans assure(s) a continued sufficient supply of cooling air.
If one of the fans in a plug-in unit fails, while the other fans continue to run, then a reverse flow of air would circulate through the stopped fan, which makes it necessary to take steps minimize this reverse flow. To this end, check valve flaps, also called non-return or anti-blowback flaps, are used, which automatically close when the air flows in the wrong direction. An example of this is disclosed in U.S. Pat. No. 6,174,232, STOLL+/IBM, which also describes the problem that the check valves used represent an additional flow resistance, and thus reduce the performance of the fan.